Select Cylinder into Equal Sections?
You could select all the outside faces, and from the header 'Select'
menu, use 'Checker Deselect', which, starting from your active face,
will skip the deselection of the number of faces set in 'Skip' and
leave a gap of N-1 faces entered in the 'Nth Selection' field, before
skipping again. Sometimes, though, that's surprisingly counterintuitive.
If you've got a lot of rotationally symmetrical work to do, it's worth considering cutting a segment out of your cylinder, and recreating the whole with an Array modifier, set to 'Object Offset', with a rotated Empty at the center of the cylinder as the modifier's target. Then, by working on one segment, you're working on them all.


But for one-off operations like this, it's often quicker to make a jig: a temporary helper object you can delete later. 

In the illustration, I've ShiftD duplicated a chunk of faces, E extruded it SZ, scaled it down in Z, and again, ShiftD duplicated it, this time immediately following up with RZ60 rotating the thickened chunk by 60 degrees. Now hitting ShiftR repeatedly will send copies round the cylinder. In this case, they're just markers to save you counting, but they could be snap targets, or view targets.



It may seem a silly amount of work, but once your favorite shortcuts are under your belt, this kind of operation for stashing lengths an angles can become very straightforward, and speed modelling up a lot.